New problems casting silver, centrifugal.

Casting | | |

In years past, I have cast some silver and gold jewelry successfully, techniques and equipment that are FAR from "by-the-book". Until this week, I had never used a vacuum de-bubbler. My burnout oven was a metal coffee can on top of a hotplate that surely never reached so much as 700 deg F (flask never glowed orange). And my centrifugal caster appears to be home-made (I bought it used), and only spins for about 30 seconds. With all this... I have cast some pretty nice stuff. But I'm trying to do better. So...

I made a vacuum de-bubbler and it really sucks the bubbles out of my investment. I bought a small burnout oven that hits 1200 deg F in 45 minutes and really makes that flask glow orange hot! I bought a nice, clean new crucible, and glazed it with Borax. All should be in order.... but my castings are having huge flaws for the first time ever. It seems to me that the metal might be flowing back out of the mold/sprue because the centrifuge doesn't spin long enough. Does that seem possible to anyone?

I melt the silver, release the centrifuge and it spins.... everything looks perfect, the metal was measured perfectly... no slop. Then, as the centrifuge starts to slow - PLOP - a glowing, red, blob of molten silver drops to the ground, and my casting looks like the one in the picture below. Also see the drip mark on the mold.

For the first time, I'm setting that flask/mold on the centrifuge glowing red hot, right out of the kiln... is that TOO hot? The metal isn't freezing fast enough? Or that is a good thing, but it needs to spin longer? At this point, it seems like just pouring it in there would do the trick. I can't afford an $800 vacuum casting machine, but given the ease with which I made a vacuum de-bubbler, that seems like something I could do as well. Thanks for any advice!

That big, ugly flaw is right where the sprue attached

More of my pics: http://www.flickr.com/photos/11168745@N06/


Rich Waugh's picture

Yep, you've analyzed the

Yep, you've analyzed the problem correctly - the flask is hot enough at the start that the metal doesn't get solidified before the centrifuge stops spinning. Obviously , there are two possible cures: either spin the centrifuge longer or let the flask cool for a few minutes before you do the casting.

Since rebuilding the centrifuge is a chore and simply waiting for the flask to cool for ten minutes is easy, that's the way I'd go. I'm lazy. :-)

As I recall, my Kerr centrifuge would spin for a bit over a minute. Perhaps you simply need to lubricate yours? Another possible cause of early stopping can be an unbalanced arm. Do you carefully balance the arm before beginning? You need to have the flask, crucible, and metal all in position and then adjust the counter weight to get the arm balanced. I usually set mine so that the counterweight end was just a tad heavier than the flask end, since the metal charge would be moving from the crucible to the flask, altering the balance point slightly. For a small casting this probably doesn't matter too much, but I regularly cast several ounces at a time and that much metal moving three or four inches can really change the balance point.

I'd suggest that you thoroughly clean and lubricate the centrifuge as a starting point. It's a simple machine that is nothing more than a spring motor and a shaft bearing with an articulated arm attached to it. Once the spring has unwound, the machine should be free-wheeling and then its all a matter of balance and good lubed bearings.

Good job on the vacuum degasser setup. I'd suggest that you wrap a layer of that fiberglass reinforced packaging tape around your "bell jar", since flat tops aren't a great idea where pressure is concerned (notice that pressure vessels have domed tops and bottoms). If the thing implodes, you don't want glass shards flying around.

Rich


Daverham's picture

Thanks for your reply, Rich.

Thanks for your reply, Rich. This has been a great opportunity to learn and experiment. Out of more-curiosity-than-desperation, I tried steam casting tonight for the first time, with the clap-the-jar-lid-over-the-flask technique. It worked great! That was fun, but more about the original problem.

I became quite convinced of my analysis just after typing it up and opted for a new centrifuge via eBay, it's on the way. Thanks for confirming, that makes me feel better!

This is the old one...

I just don't have faith in that old thing, it spins maybe 5 or 6 rotations and then stops. I certainly won't throw it away. I did lubricate and clean it, and did a quick balance job, although not perfect. It just doesn't quite go long enough. There are no bearings, just a steel rod in a bronze bushing. I think it's an old DIY project, like my degasser :)

On that note, thanks for the tip on the fiberglass.


bpfink's picture

Casting sprue

Rich W below makes some really good points.  One other minor thing I might add is to make a small crimp cut in the wax sprue above the piece and below the cup before the investing.  Since the cup is the least insulated after the pour it will be the first to cool.  In doing so it shrinks and will have the tendency to pull a little metal back out of the lower and next bigger portion (that would be the ring body) and that would cause a tear or break or flaw in the inside body.  It is a simple way to stop that possibility as the crimp allows the sprue to then part from the body of the piece.   bpfink


Rich Waugh's picture

Great suggestion on the

Great suggestion on the crimp cut in the sprue, Bruce! I'd completely forgotten that since it was something we used doing sculpture but I ever had a need for doing jewelry.

Daversham,

That is clearly a homemade casting machine and it is not made correctly at all. The arm is fine enough, but the spring motor is not correct - you need a flat coil spring (like clock spring) to allow the machine to continue to provide impetus beyond the first revolution. That arrangement on the homemade machine will only supply impetus for the first three quarters of a revolution and that's not near enough.

I hope the new one works great for you.

Rich


Mark Ruby's picture

Casting

If you send me your snail-mail address I will send you a work sheet made up by one of my students. It gives you a (almost) foolproof set of instructions for casting jewelry. Flask temp should be about 1000 F. I agree with the other comments that the homemade casting machine ain't gonna cut it.


Rich Waugh's picture

I want to clarify what I

I want to clarify what I said about that homemade centrifuge not doing the job: I was speaking of only that particular one, due to its inadequate spring motor.

Homemade centrifugal casting machines can work every bit as well as the commercial ones, provided that they're made correctly. I've made one that was electrically powered and worked terrifically for particularly large jewelry/small sculpture castings of up to about a kilogram of metal. I made an articulated arm virtually identical to the one used on my Kerr centrifuge, but powered it using the starter motor from a junked car. The starter motor provided instant high torque and actually developed acceleration for the first half second or so before it was switched off and allowed to coast down. The bearings in the starter motor were very smooth and allowed it to coast for some three minutes before it came to a stop. The main drawback was that it required a battery to power it and a charger for the battery. Still, I used it completely successfully for a few years until I sold it.

With any casting centrifuge the single most important part is the scatter shield. Nothing will ruin your weekend like a shower of molten silver across your midsection!

Rich


Daverham's picture

Thanks for a good

Thanks for a good discussion. I have since ordered a "real" casting machine and look forward to trying that and reporting my results.

For the record, I HAVE used my little home-made unit for successful castings many times - silver, gold and copper - but that was due to a lucky alignment with other flaws in my process: My (also home made) burnout oven didn't get very hot, probably just barely melted the wax out, to be honest. That was an asbestos-lined, upside-down coffee can on top of a hot plate. With a not-hot-enough flask and a too-slow centrifuge it actually worked!

Of course that is the source of other problems, my investment not really curing (I broke a couple during casting), potential for incomplete burnouts, especially when using patterns made of things other than wax, etc.

Looking back, I'm amazed I have ever cast anything with that setup, but it did work. And the guy I got it from (Walter Tussinger of Ellensburg, Washington) used it for many years to create stunning hand-made, gold and silver jewelry - both the home-made burnout oven and the casting machine. I bet he made them himself. He showed me how to cast, out of the kindness of his heart, and when he died, about 10 years ago, his lovely wife, Meriam, gave me his casting equipment - exactly as-is, and just how it was when he showed me how to use it. Anyway, it can be surprising what works and what doesn't.... and why.

Only since I upgraded my kiln did I start having this problem. Before that, the flask was cool enough to freeze the metal at just the right time!

Yes... I am happily moving on to a more reliable and up-do-date system: a real kiln and a real casting machine - and hanging on to Walter's gear for nostalgic reasons. I didn't even really know Wlater and Meriam very well, I just wandered into their shop and started asking questions. What a great couple! By the way, the new owners of that shop (Ellensburg Agate Co.), Steve and Unetta, are just as wonderful, although no free casting equipment ;)

I know what you're saying... with a properly-heated flask that casting machine won't work. That much has been proven. I'm excited to get my new one and do some great work. Can't wait to share.


Daverham's picture

Back in business! My new

Back in business! My new centrifuge is working very well. Last night I whipped up a little wax pattern as quickly as I could muster and tested the new centrifuge. Works very well.

now... time to buckle down and design/craft some REAL works of art. I'm all geared up and excited to get to it. Thanks for all the help/input along the way.

More pics: http://www.flickr.com/photos/11168745@N06/


Rich Waugh's picture

Okay, you're all set now. I

Okay, you're all set now. I see that they've added a couple things to the basic casting machine since I got mine forty years ago - looks like it now has a bir red handle to wind/release it and a plastic cover on the set pin. That's not a lot of changes for that much time, is it? Tells you that it is a basic, sound design that needs no modification, I guess.

We'll be awaiting pictures of your new creations. Get busy! (grin)

Rich


Daverham's picture

Yessir! I have two half-made

Yessir! I have two half-made wax patterns at home calling my name. That reminds me: a sort-of-philosophical question. I'd like to make the most of my supplies, what do the pros do? Does anybody cast just one piece at a time, or do you wait until you have a whole sprue-tree's worth? I guess it's up to me, just curious what's "best practices". Seems like a trade off between efficiency and the risk that if one thing goes wrong, all is lost, lots of eggs in one basket, etc. Maybe I'll just sprue up two or three things at a time.

About that red knob: I had not seen the before either, but to tell you the truth, it's pretty nice to grab hold of that and wind 'er up!


Rich Waugh's picture

I always used to try to get

I always used to try to get as much into a flask as possible, and only did one-off castings when I had a solo commission that had to be done right away. Every time you melt the metal you lose a tiny bit of alloying ingredients so I tried to keep the number of melts to a minimum. With gold, after a few melts I wold assay the sprue button and alter the chemistry if necessary to maintain the karat value.

Rich


visitor's picture

Casting in general.

First, I'd raise the wall of the wood on that casting machine I see, at least 6 inches, you have hot molten metal hit you at over 1700'F it can seriously hurt you or worse. Second you need to get you flask temperature up as close to 1350'F for at least 30 min, to make sure all your wax has been burnt out then back down to around 1000'F to cast, to hot and it stays molten to long and is again very dangerous. Depending on the sizes of you wax originals and flasks would make the decision as to how many waxes to each flask. If you are casting all the same metal and roughly the weights then go head and put more, depending on your flask size, into each flask. And third, remelting the same metal shouldn't be to much of a problem as long as you try to use as close to AT LEAST, 50% new to used each time you cast. Same with silver etc. Don't try to cast something like platinum unless you have the right equipment, that will go right through you due to it's liquidous temperature, it can kill you!


visitor's picture

The number of waxes you

The number of waxes you should put in a flask is going to depend more on the size of the crucible you are using. That determines capacity. If all can melt is 1 troy ounce than that's it. bj


ak1008's picture

Hi! i have been reading some

Hi! i have been reading some of your posts. Found my way here browsing steam casting information.

 

All i have at the moment is a torch and waxes and want to start casting.  I was thinking to try using steam casting method since the centrifugal casters are very expensive here in sweden. But i have found these dental centrifrugal machines on ebay that looks like yours that is way cheaper..different brands around 150-200$.

But they do seem to be alot smaller in size. What are the dimentions on this caster you bought and are you still happy with it?

 

 

thanks /adam k